Continuous feed centrifuges are used in many industrial applications for separation of solids and liquids. In general, such continuous feed centrifuges include an outer rotating member in the form of a solid or perforate bowl. Examples of continuous feed centrifuges are disclosed in commonly assigned U.S. Pat. Nos. 4,381,849; 4,464,162; 5,147,277 and 5,378,364. As used herein, continuous feed centrifuges include sedimenting solid bowl and filtering pusher and scroll/screen as well as hybrid sedimenting and filtering screen bowl centrifuges. For ease of illustration, the present invention will be primarily described from the standpoint of a solid bowl centrifuge and therefore the components and operation of prior art solid bowl centrifuges will now be described in some detail.
A solid bowl or decanter centrifuge generally includes an outer bowl, an inner hub carrying a scroll conveyor, a feed compartment within the conveyor wherein the feed slurry is accelerated to speed before being introduced into the separation pool, and discharge ports for cake solids and clarified liquid or centrate. It will be appreciated that the cake solids will be interchangeably referred to herein as solid, heavy phase or higher density discharge or output stream. Similarly, the clarified liquid or centrate will be interchangeably referred to herein as liquid, light phase or lower density discharge or output stream. The bowl includes a cylindrical section and a conical beach section. The bowl and the hub are rotated at high, angular speeds so that heavier solid particles of a slurry, after accelerated to speed and introduced into the bowl, are forced by centrifugation into an annular layer along the inside bowl surface thereof. By differential rotation of the scroll conveyor and the bowl, the sediment is conveyed or scrolled to a cake discharge opening at the smaller, conical end of the bowl. Additional discharge openings are provided in the bowl, usually at an end opposite of the conical section for discharging a liquid phase or liquid phases separated from the solid particles in the centrifuge apparatus.
Controlling and optimizing the operation of such centrifuges is a difficult task considering the high rotational speeds of the bowl and hub, and the continuously changing characteristics of the input or feed stream (slurry) and the light phase and heavy phase output streams. Notwithstanding these difficulties, there have been some attempts in the prior art to provide control systems for bowl/conveyor type (decanter) centrifuges. For the most part, all of these control systems utilize torque measurement (e.g., dc or steady torque measurement) as an input for controlling the speed of the conveyor and/or bowl. Examples include U.S. Pat. Nos. 4,369,915; 4,432,747 and 4,668,213. All of these patents disclose a torque measuring device for measuring the torque input to the screw conveyor and based on this torque measurement, the differential speed between the bowl and conveyor is optimized. In U.S. Pat. No. 5,156,751 to Miller, a similar type of centrifuge is shown wherein sensing and control means 33 regulates the speed of the conveyor 22, the control means being responsive to a torque measurement.
U.S. Pat. No. 4,303,192 ('192) to Katsume discloses a centrifuge control system which controls and/or regulates the differential speed between the bowl and the conveyor and/or the solid matter quantity supplied to the centrifuge per unit of time in response to the sensing of certain operating parameters such as (1) the torque of the conveyor and/or (2) solid matter concentration in the solid matter discharge and/or (3) solid matter concentration in the liquid separation product discharge. The '192 patent discloses a measuring unit 43 for measurement of torque, a solid matter concentration measuring unit 40 for measurement of the centrifuge solids discharge and a solid matter concentration measuring unit 38 for measurement of solids concentration in the liquid discharge. Measuring unit 40 determines the quantity and/or the solid matter concentrations of the concentrated sludge being output and converts the resulting value into an electrical signal. Similarly, the solid matter concentration in the liquid separation product is determined by measuring unit 38, converted to an electrical signal and transmitted to computational unit 42, 48. As stated in column 6 of the '192 patent, lines 24-33, the control system has three input variables including (1) torque of the conveyor, (2) quantity and concentration of solid matter in the solids discharge and (3) quantity and concentration of solid matter in the liquid separation product. Based on this input, three controls of the centrifuge are initiated including (1) the speed of the bowl, (2) the differential speed of the bowl and conveyor and (3) the amount of solid matter/slurry quantity being supplied to the centrifuge.
Other decanter centrifuge patents describing control systems include U.S. Pat. Nos. 5,203,762 ('762) and 4,298,162 ('162). The '162 patent describes a control system for controlling the drive motors of the centrifuge using several ac/dc conversions for generating power from the backdrive motor and converting this power for use by the main drive motor. The '162 patent utilizes a gear which interconnects the screw conveyor to the bowl and two rotary, positive displacement machines for controlling relative rpm of the conveyor.
Unfortunately, none of the aforementioned prior art provides a comprehensive computerized (e.g., microprocessor) control system for operating, controlling and monitoring continuous feed centrifuges such as solid bowl, screen bowl, scroll/screen or pusher type centrifuges. However, the ability to provide precise, real time control and monitoring of such centrifuges constitutes an on-going, critical industrial need.